Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Consequently, the use of exhaust after-treatment systems on engines to reduce emissions is increasing.
Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx). Diesel oxidation catalysts (DOC) have been implemented in exhaust gas after-treatment systems to oxidize at least some particulate matter in the exhaust stream and to reduce the unburned hydrocarbons and CO in the exhaust to less environmentally harmful compounds. To remove the diesel particulate matter, a diesel particulate matter filter, or PM filter, is typically installed downstream from the DOC or in conjunction with the DOC.
A common PM filter comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. The common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter. Accumulation of particulate matter in the PM filter causes backpressure to build within the exhaust system. Excessive backpressure on the engine can degrade engine performance (e.g., lower power and efficiency), and in some cases may result in engine stall.
Particulate matter, in general, oxidizes in the presence of nitric oxides (particularly NO2) at modest temperatures, or in the presence of oxygen at higher temperatures. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may get high enough to cause an uncontrolled high-temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process. To prevent potentially hazardous situations, accumulated particulate matter is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated.
For oxidation of the accumulated particulate matter, exhaust gas temperatures generally must exceed the temperatures typically reached at the filter inlet. Consequently, additional methods to initiate regeneration of a diesel particulate filter may be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to increase the temperature of the exhaust gases entering filter and initiate oxidation of particulate buildup. A PM filter regeneration event occurs when substantial amounts of soot are consumed on the PM filter.
A controlled PM filter regeneration can be initiated by the engine's control system when a predetermined amount of particulate has accumulated on the filter, when a predetermined time of engine operation has passed, or when the vehicle has driven a predetermined number of miles. Controlled or active PM filter regeneration typically consists of driving the filter temperature up to O2 oxidation temperature levels for a predetermined time period such that oxidation of soot accumulated on the PM filter takes place.
With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx, conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems utilize a spray dosing system to introduce ammonia reductant into the exhaust stream upstream of the SCR catalyst. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if excess ammonia is introduced into the exhaust stream upstream of the SCR catalyst, the surplus ammonia can slip through the SCR without being utilized in the catalytic reaction. Because ammonia is an extreme irritant and an undesirable emission, ammonia slips of even a few tens of ppm are problematic. As a result, an additional ammonia oxidation (AMOX) catalyst may be installed downstream of the SCR catalyst to capture and oxidize any residual ammonia components in the exhaust stream into more benign compounds prior to being released into the atmosphere.
Due to the undesirability of handling pure ammonia, many systems utilize an alternate solution, such as aqueous urea, that vaporizes and decomposes to ammonia in the exhaust stream. However, the decomposition of urea into ammonia can be highly dependent upon the temperature of the exhaust gas. If the temperature of the exhaust gas is too low, for example, some of the vaporized urea can condense onto the interior surfaces of the SCR system piping and the SCR catalyst bed and crystallize into urea deposits, which can build over time.
Like the build-up of particulate matter in the PM filter, the urea deposits in the SCR system may also have negative impacts on the operation of the internal combustion engine. For instance, the deposits may restrict the flow passages of the exhaust stream, causing higher back-pressures and reducing engine and after-treatment system performance and efficiency. The deposits may also disrupt the flow and mixing of the urea reductant into the exhaust stream and thereby reduce the decomposition into ammonia (NH3) with a subsequent drop in NOx reduction efficiency. Moreover, the re-direction of a portion of the injected urea into the urea deposits can also reduce the amount of urea reductant which was intended to reach the SCR catalyst, making control of the SCR system more difficult and further reducing NOx reduction efficiency.
Similar to the periodic controlled oxidation or regeneration of the PM filter to remove an excess accumulation of particulate matter, an increase in the temperature of the exhaust gases entering the SCR system can release and decompose the urea deposits into ammonia for utilization or storage in the SCR catalyst. However, if the urea deposits have been allowed to grow too large, the additional surge of ammonia can be in excess of the amount which can be effectively utilized in the ongoing catalytic reaction or captured and stored in the SCR catalyst, and thus will escape the SCR system as ammonia slip. In addition, the repeated cycling of the SCR system inlet temperature to temperature levels sufficient to release and decompose the urea deposits into ammonia can both reduce the efficiency of the ongoing SCR reaction as well as cause premature wear and degradation of the SCR catalyst components, and should be limited as much as possible.
Also, many known SCR systems do not utilize an ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst to convert at least some ammonia slipping from the SCR catalyst to N2 and other less harmful compounds. For those conventional SCR systems that do not include an AMOX catalyst, the operating conditions and conversion capability of the AMOX catalyst are not factored into the reductant dosing rate, ammonia storage control, ammonia slippage control, and NOx conversion efficiency feedback of such systems. However, these conventional AMOX-less SCR systems are still required to comply with the stringent emission requirements for internal combustion engines, and must meet these standards through different means.